Wireless devices increase their usefulness with each standardized communication channel on which they can operate. Often, operation on multiple communication channels requires operation on different frequencies bands. For example, 802.11 is grouped into multiple bands of operation. An antenna that operated on 2 of the bands (i.e, dual band) would be more valuable than a single frequency antenna. Further, a tri-band (3 bands) would be more valuable than a dual band.
Communication frequency bands may overlap or be in sufficiently close proximity that the effect is a wider bandwidth than any one communication channel. Also, wider bandwidths are necessary for some high data rate transmissions, such as video streaming and the like.
To accommodate these wider bandwidths and multiple communication channels, many wireless devices have incorporated multiple antennas. While this works, it increases the complexity of the wireless device, the size of the wireless device, and the cost to manufacture the wireless device. Another solution would be to provide a log periodic antenna, but log periodic antennas generally require fairly large structure with multiple elements.
One common antenna useful to operate across multiple bands is a planar inverted F antenna (PIFA). PIFAs provide a good match (without a matching network) at different frequencies simultaneously to allow multiple band operation. However, when bands are close together in frequency, the match becomes difficult to achieve.
Another problem with the PIFA is that as the size of the PIFA is reduced to accommodate smaller and smaller handheld style devices, the bandwidth of the PIFA shinks as well. In other words, the minimum bandwidth of a PIFA often limits the maximum size reduction. An important measure of antenna bandwidth is called percentage bandwidth, or PBW. PBW is computed asPBW=(fu−fl)/(√fufl)×100  equation #1In equation #1, fu is the upper frequency of the bandwidth. fl is the lower frequency of the bandwidth. For the typical handheld wireless device, most PIFAs have a 10% PBW.
Thus, it would be desirable to develop a multi-band antenna having a wide bandwidth.